fix: use full chord type for roman-numeral sevenths in Add harmony dialog

[FelipeDefensor]

Warning

AI-generated — needs verifying. This investigation, fix, and tests were produced by an AI agent (Claude Code) and have not been independently verified by a human. Please review carefully before merging — see the checklist at the bottom.

Summary

Typing a roman-numeral seventh chord (e.g. V7) in the Add harmony dialog selected the triad quality (Major) instead of the seventh quality (Dominant seventh). Reported against v0.6.3.

Root cause

The roman-numeral parse path in _get_params_from_music21_object (tilia/timelines/harmony/components/harmony.py) read music21's RomanNumeral.impliedQuality, which only reflects the triad quality (major/minor/diminished/augmented) and ignores the seventh/extension. So V7 resolved to major, and the dialog's quality combobox selected "Major".

This is a regression from commit 7a76fdc ("chore: more accurate naming"), a chord_* → letter_* rename. It correctly renamed the set side (roman_numeral.chord_type → roman_numeral.letter_type), but on the read side changed obj.chord_type to obj.impliedQuality instead of obj.letter_type. That left letter_type set-but-never-read, and the reader silently picked up music21's triad-only attribute.

letter_type is computed from the chord's actual pitches (Chord(roman.pitches).commonName → CHORD_COMMON_NAME_TO_TYPE), so it carries the correct full chord type.

Fix

One line:

-        quality = obj.impliedQuality
+        quality = obj.letter_type

Tests

Adds TestRomanNumeralParsing to tests/ui/dialogs/test_select_harmony_params.py. It drives the dialog the way a user types (char-by-char into the line edit via the existing parse_text helper) and asserts on the resulting combobox selection — i.e. it exercises the exact reported symptom, not just the parse function.

Cases: V (triad, unchanged), V7, V65, V43, V42, ii7, viio7, and the applied chord V7/V. Confirmed these fail on the buggy code (assert 'major' == 'dominant-seventh') and pass with the fix. Full harmony suite (171 tests) green; ruff clean.

Reviewer checklist (AI output — please verify)

• Music-theory correctness of the expected qualities/inversions for the parametrized roman numerals.
• letter_type is always set whenever the parse path returns kind == "roman" (it is assigned inside the try block in _get_music21_object_from_text).
• No other reader relies on the previous impliedQuality behavior for roman numerals.